At first, the job seemed difficult, but I quickly got used to it.

at 是介词，表示"在...时候"或"处于...状态"。 first 是序数词，意思是"第一"或"最初的"。 at first 是一个固定搭配的短语，意思是"最初"或"刚开始的时候"，用来描述事情刚开始的状态。